Bus / Tram Signal Priority

Studies worldwide have shown growth in public transport passenger patronage as a result of measures which set effective traffic priorities. In the US passenger numbers along commuter corridors equipped with bus rapid transit systems - increase by an average of 35% according to the US Department of Transportation’s Federal Transit Administration. Bus rapid transit - defined as bus public transport enhanced with ITS systems for better services - is winning new passengers wishing to avoid personal car transport and the associated fuel costs and traffic congestion. Public transport vehicles can be given priority over general traffic by integrating their operation into urban traffic control (UTC) systems. Automatic Vehicle Location (AVL) enables buses and trams to be identified as they approach signalised intersections, where they transmit a ‘request’ to the traffic light controller to extend or recall the green phase for long enough to let them through. Detection can be via inductive loops under the road surface, roadside beacons, or GPS systems, which may be integrated with real-time information systems.

Another priority system is the guided busway, which has been implemented in Germany, Australia and the UK. This supplements conventional bus lanes with specially-designed track sections. There are both mechanical and electronic systems. In electronic systems an electric cable is embedded in the centre of the busway. On-board inductive detection steers the wheels continuously to keep the vehicle centred over the cable. At the end of a busway section, traffic signal priority allows access to general roadway lanes.

Inductive loop detectors can be used to detect the passage of vehicles in a given location. The detector consists of a wire loop embedded in the surface of the roadway which is connected to an electronic unit housed in a controller cabinet. The presence of a conductive metal object is sensed as a reduction in loop inductance - which is ultimately interpreted by the controller as a vehicle. While this is a commonly used technology, virtual GPS systems have now entered the market – and these may be linked to the on-vehicle computer.

Such systems and infrastructure for controlling accessing to a busway need to be integrated and co-ordinated with other elements of the road network operator’s asset base.

ADVICE TO PRACTITIONERS

It is essential that the road network operators work together with transport operators to ensure that ITS systems are both designed to function together and also actually do in practice. The aim is to enable the provision of reliable bus services and in congested areas this may require a policy of prioritising buses over other classes of traffic. Any consequent negative impacts on other classes of road users should be thought through and not just occur as an unplanned result of actions, policies or systems for public passenger transport.

Areas of which can cause difficulty include communications protocols, compatibility of infrastructure, proprietary systems, data transfer, incompatible location referencing, lack of open standards, and operational procedures. Responsibilities of each organisation also need to be clearly understood as part of the concept of operations when developing the ITS architecture. (See ITS Architecture)

It is essential that all parties communicate and partner together effectively over street layout and the choice of traffic control equipment. Equipment that is purchased by road network operators must be compatible with standard in-vehicle detection and activation equipment installed by bus manufacturers.

Sub-surface detector loops are not capable of distinguishing between different vehicles of the same type and so are not suited to monitoring the location of a specific vehicle. Some traffic control systems are capable of allowing selective bus or tram signal priority, depending on whether or not the vehicle is running late.

WHAT’S NEW?

Increasingly, AVL and communications with road infrastructure are being integrated - leading to a reduction in the number of on-board bus computer units.

ISSUES FOR DEVELOPING ECONOMIES

Wherever a culture of low adherence to traffic rules exists, in order to ensure effective traffic priority, the visible presence of traffic supervisors to enforce rules may be necessary irrespective of the presence of ITS systems and traffic signals. In such circumstances the traffic supervisors need training on working effectively with the ITS systems.